In digital communications systems, digital information to be transmitted is used to modulate the amplitude, angle (i.e., phase or frequency) or both of a carrier signal. The resulting modulated carrier signal is then used to carry the digital information across a communication channel to a receiver. Whether the digital information is used to modulate the amplitude, the angle, or both the amplitude and angle of the carrier signal depends on what modulation scheme is employed. In some modulation schemes only the angle of the carrier signal is modulated, so that the resulting modulated carrier signal has a constant-amplitude, i.e., a “constant envelope.” Such is the case with Gaussian minimum shift keying (GMSK), which is the modulation scheme used in second generation (2G) Global System for Mobile Communications (GSM) cellular communications systems. In GMSK a continuous-phase frequency-shift keying type of modulation scheme is employed which affects only the frequency of the carrier signal.
To increase spectral efficiency, many state-of-the-art communications systems, including third generation (3G) cellular communications systems such as EDGE (Enhanced Data rates for GSM Evolution) and W-CDMA (Wideband Code Division Multiple Access), employ modulation schemes that produce non-constant-envelope signals. To prevent signal clipping of these types of signals in traditional quadrature-modulator-based transmitters, the levels of the signals are reduced before being introduced to the transmitter's power amplifier (PA), and the PA is configured to operate in its linear region of operation. Unfortunately, this results in a very inefficient use of power and a trade-off between linearity and efficiency.
The linearity versus efficiency tradeoff experienced by conventional quadrature-modulator-based transmitters can be avoided by using a polar modulation transmitter. In a polar modulation transmitter, amplitude and phase modulation are processed in separate amplitude and phase paths. An amplitude modulation (AM) signal representing envelope information is used to generate an amplitude modulated power supply signal in the amplitude path, while a constant-amplitude phase modulation (PM) signal is used to modulate a carrier signal generated by a voltage controlled oscillator (VCO) in the phase path. The phase-modulated carrier signal at the output of the VCO is amplified by the polar modulation transmitter's PA. Because the phase-modulated carrier signal has a constant amplitude, the PA can be configured to operate as a highly-efficient nonlinear PA without the risk of signal clipping. Typically, the PA comprises a switch-mode type of PA (e.g., a Class D, E or F PA) configured to operate in compression, so the amplitude modulation in the amplitude modulated power supply signal is directly modulated onto the phase modulated carrier signal as the phase modulated carrier signal is being amplified. Hence, by processing the AM and PM signals in separate paths, the traditional trade-off between linearity and efficiency is avoided.
In addition to being power efficient, the polar modulation transmitter is adaptable to different modulation schemes using digital signal processing techniques. Digital signal processing allows the same radio architecture to be used for different standards, thereby making the polar modulation transmitter well-suited for multimode operation. By contrast, conventional quadrature-modulator-based transmitters require the use of narrowband surface acoustic wave (SAW) filters to attenuate spurious signals generated by upconverting mixers used to translate the baseband signal to RF. However, because each modulation standard typically operates under a different bandwidth, providing multimode capability becomes expensive and difficult to accomplish in quadrature-modulator-based transmitters.
Although the polar modulation transmitter offers a number of advantages over conventional quadrature-modulator-based transmitters, it does present some challenges. One important challenge relates to the fact that the modulation signals are processed in the polar domain (i.e., in terms of polar coordinates). Modulation signals processed in terms of polar coordinates typically have higher bandwidths compared to modulation signals processed in terms of in-phase and quadrature (i.e., rectangular) coordinates. Depending on the modulation scheme being used, signal trajectories can pass very close to the origin of the complex signal plane. Generally, these low-magnitude events correspond to changes in the phase of the PM signal occurring in a short interval of time. For example, for signal trajectories that pass through the signal plane origin, instantaneous phase changes of + or −180° can occur.
Unfortunately, the polar modulation transmitter's VCO is unable to respond to large phase changes that occur in short intervals of time. VCOs are comprised of capacitors and inductors, which are by their very nature unable to respond to rapid phase changes. They are also phase-accurate only over a narrow bandwidth. Accordingly, when subjected to PM signals having rapid phase changes, the VCO is unable to produce a phase-accurate output. Phase inaccuracies are highly undesirable since they result in data transmission errors and increase the error vector magnitude (EVM) at the receiving end of the system, thereby making it extremely difficult to comply with communications standards specifications.
It would be desirable, therefore, to have methods and apparatus for reducing or removing rapid phase changes in the phase path of a polar modulation transmitter while, at the same time, maintaining the ability of the polar modulation transmitter's VCO to provide a phase-accurate output.